1. Field of the Invention
The present invention relates implantable electronic medical devices, such as cardiac pacemakers and defibrillators for example, for stimulating tissue of animal for the therapeutic purposes, and such implantable medical devices that are compatible with magnetic resonance imaging (MRI).
2. Description of the Related Art
Numerous medical conditions, such as cardiac and neurological dysfunctions, are treated by an implanted electronic device which provides electrical stimulation to the affected tissue of the animal. These devices have a plurality of metal components, including the generator case and wire leads extending from the case to electrodes in contact with the tissue to be stimulated or monitored.
Magnetic resonance imaging (MRI) is commonly employed to view internal organs of medical patients. To create an image, the patient is placed into very strong static and varying magnetic and radio frequency (RF) fields and thus MRI generally is prohibited for patients with implanted ferromagnetic and or electrically conductive objects. Although it is feasible to minimize and even eliminate the use of ferromagnetic materials in implanted apparatus, electronic devices, such as cardiac pacemakers and defibrillators, require electrically conductive components that are affected by the fields produced by an MRI scanner.
It has been a long-standing goal to make implanted devices MRI compatible so that this imaging modality can be used with patients having those devices. There are several reasons for achieving this goal. First, incompatible implant components induce susceptibility difference, which destroys DC magnetic field homogeneity, thereby affecting the imaging performance of the magnetic resonance scanner. Second, conductive materials present an opportunity for eddy currents to form, which currents generate heat that adversely affects patient safety and degrade the scanner performance by field distortion. Third, the MRI fields may ruin the implanted device. Fourth, the incompatible implant material can potentially cause serious internal injuries to the patient.
The issue of MRI interaction with electronics of an implanted device has to be considered in an integrated fashion to provide compatibility. Table 1 shows combinations of interactions that are briefly discussed hereinafter.
TABLE 1Interactions of Factors Influencing MRICompatibility of an Implanted Device or ComponentEffect on theEffect on thePatient SafetyImplanted DeviceMR ImageDC MagneticIIIIIIFieldsGradientIVVVIMagnetic FieldsRF FieldsVIIVIIIIX
I. Any ferromagnetic material inside the implanted device exposed to the MRI fields experiences a force and a torque, the amount of which depends on the shape, dimensions, and amount of ferromagnetic material. The forces are greatest in areas where there is a gradient in the magnetic field, e.g. upon entering a MRI system. Obviously the surrounding tissue adjacent the implantable device will be damaged in this case and the health of the patient will be compromised. In addition, metallic components can become hot and burn the patient.
II. Due to MRI field induced torque and movement of the implant, its components may become disconnected making the device inoperable. Ferrites and other ferromagnetic material in transformer cores, inductors and other electronic components become saturated, thereby jeopardizing the function of the medical device. Heating causes electronic components to operate out of specification.
III. The homogeneity of the magnetic resonance imager's DC magnetic field will be distorted, destroying spectral resolution and geometric uniformity of the image. The inhomogeneous field also results in rapid de-phasing of the signal inside the excited volume of the patient. The resultant image shows a distorted view of the patient's anatomy.
Even if the implanted device does not contain any ferromagnetic materials, the magnetic susceptibility of the device may be different than that of the surrounding tissue, giving rise to local distortion and signal dropouts in the image, close to the device. This is especially true for pulse sequences that are sensitive to phase, like echo planar imaging
IV. Switching field gradients create large eddy currents, at frequencies up to a few kilohertz, in the metallic housing of an implantable device and any metallic part that forms a loop, such as cables forming a loop. These eddy currents make the device move with the same frequency as the leading and trailing edges of gradient pulses. This movement can be unsafe for the surrounding tissue. The associated eddy current pattern creates local pulsating E-fields, in addition to the E-field generated by the MRI scanner's gradient coil, which can stimulate the patient's nerves. Resultant muscle twitching can be so intense as to be painful.
V. The eddy currents may be strong enough to damage electronic circuits and destroy the implanted device. The pulsating forces on the device may disconnect components.
VI. The eddy currents affect the rise time of the MRI gradient pulses, and therefore affect the minimum obtainable echo time, necessary for many pulse sequences. The eddy currents also locally distort the linearity of the gradient fields and de-phase the spin system, resulting in image distortion and signal dropouts. Phase and frequency encoding of the signal strongly depends on the linearity of the gradients.
VII. The RF field interacts with any metallic part in the device, be it either in the form of a loop, which results in B-field coupling, or a straight conductor, which results in E-field coupling. The B-field component of the RF field can induce currents and voltages in conducting loops. The amplitude depends on the impedance of the loop at the RF frequency, and the size of the loop. An example may be two coaxial cables that form a loop together. Such a loop may have high impedance at DC due to the insulating outer shell of the coax, but the distance between the cables at the crossover point may be equivalent to just the right amount of capacitance to make the loop resonant at the RF frequency.
The E-field component of the RF field will induce voltages and currents in straight conductors, like a single cable for example. The amplitude of the induced voltages and currents depends on the phase length of the conductor, or path, at the associated radio frequency.
The induced voltages and currents create locally very strong E-fields that can burn the patient.
Non-metallic implantable devices do not have these issues, but can still distort the uniformity of the RF field if the permittivity of the device is different than that of the surrounding tissue. This distortion is especially strong at radio frequencies above 100 MHz.
VIII. Localized high voltages and currents in the medical device may cause components to fail either due to high voltage arcing, or due to dissipated power and heat. This includes connections that become unsoldered due to the heat. The device may generate pulsed voltages at unwanted times and locations in the leads of a cardiac pacemaker.
IX. Local distortion of the uniformity of the B-field component of the RF field will give rise to flip angle variation and creates contrast and signal-to-noise ratio (SNR) inhomogeneity. The specific absorption rate, which is defined as the RF power absorbed per unit of mass of an object, can exceed legal limits. If the specific absorption rate exceeds legal limits, images cannot be made using magnetic resonance scanners.
From a fundamental physical perspective, it is useful to examine the conductivity of wires at high frequencies of MRI. As frequencies increase, conduction begins to move from an equal distribution through the conductor cross section toward existence almost exclusively near the surface. Depending on the conductor bulk resistivity, at sufficiently high frequency all the RF current is flowing within a very small thickness at the surface. Lower bulk resistivities result in shallower skin depths.
For a solid wire, the current concentrates on the outer surface. For this reason, when skin depth is shallow, the solid conductor can be replaced with a hollow tube with no perceivable loss of performance. Choice of a plating material can degrade performance (increase attenuation) if its bulk resistivity is greater than that of the body of the wire. If such a conductor is placed inside the E field of an MRI RF Transmit coil, there will be RF energy deposition in the tissue surrounding the wire resulting in elevated temperatures that may result in physical injury to the patient.
Therefore it is desirable to provide an integrated, comprehensive solution to MRI compatibility of an implanted medical device that contains electronics circuitry. Furthermore, it is desirable to make leads that connect implanted medical devices to the electrodes for stimulation therapy and/or electrical sensing MRI compatible.